Circulator Sizing for Indirect

I'm trying to finish specing out the components I'll need to convert my tankless coil to an indirect water heater. At the moment I am trying to properly size the circulator pump. I was hoping someone could check my work. All calculations were taken from the article on circulator sizing on this web site located here: http://www.comfort-calc.net/circulator_sizing.html.

You're probably going for overkill here- unless you have enough boiler output that you need higher flow to protect the boiler there's no advantage to higher flow rate. Install enough pump to keep the boiler's delta-T down to the 20-25F range and you'll be getting about as much output out of the system as you would at higher flow, which would only result in a lower delta-T on the boiler, not more first-hour gallons out of the system.

If your boiler's output is, say, 108,000BTU/hr and you're designing for a 20F delta-T, that means you need 108K/20=5400lbs/hr of water pump rate. A 8.34lbs/gallon that's 5400/8.34=647gallons/hr which is 10.8gpm. For a 25F delta-T you only need 8.6gpm, and for a 30F delta-T, 7.2gpm.

As the flow rate drops the head also falls, and it's faster than linear- at 7gpm the head is roughly only about half what it is at 10gpm. At 8.5gpm it's about 70-75% of what it is at 10gpm, so a Taco 007 would be more than enough pump to keep the delta- under 30F, and it would run close to 25F. A Taco 0010 would lower the delta to close to a bit over 20F. A Taco-0012 would be overkill, raising the flow to something like 14-15gpm, and the delta-T down to ~15F for no benefit.

Methinks a Taco-007 would be fine here. You'd be giving up at most a few percent of first-hour gallon performance due to lower-than-recommended flow on the SS. If you're concerned that the boiler is ultra-sensitive to high delta-T, go with a Taco-0010, then kick yourself when it turns out your head calc ran a bit high.